A polycrystalline silicon ingot has been used as a material for a solar cell substrate, for example, as disclosed in Patent Document 1. Namely, the polycrystalline silicon ingot is cut into slices having a predetermined thickness to make a polycrystalline silicon wafer, and then the polycrystalline silicon wafer is processed to produce a solar cell substrate. Performances of the solar cells, such as conversion efficiency may depend a great deal on characteristics of the polycrystalline silicon ingot, that is, the material for the solar cell substrate (polycrystalline silicon wafer).
In particular, higher amount of oxygen and impurities in the polycrystalline silicon may lead to a significant decrease in the conversion efficiency of the solar cell. Thus, it is necessary to reduce the amount of oxygen and impurities in the polycrystalline silicon which acts as the solar cell substrate to maintain the conversion efficiency of the solar cell in a high level.
The polycrystalline silicon ingot unidirectionally solidified in a crucible, which is obtained by sequential solidification in a predetermined direction, tends to have increased amounts of oxygen and impurities both at the bottom, a starting point of solidification, and the top, an end of solidification. Thus, both the bottom and the top of the polycrystalline silicon ingot unidirectionally solidified should be cut off and removed to reduce the amount of oxygen and impurities.
The reasons why the amount of oxygen and impurities increase at the bottom and the top of the polycrystalline silicon ingot will be described in more detail below.
When a silicon melt is unidirectionally solidified in the crucible upwardly, the impurities migrate from a solid phase to a liquid phase in the silicon melt because the impurities are less soluble in the solid phase than in the liquid phase. While this results in a decreased amount of impurities in the solid phase, it leads to a significantly increased amount of impurities at the top of the polycrystalline silicon ingot, i.e., at the end of solidification.
When the silicon melt is left in a silica crucible, oxygen is incorporated into the silicon melt from silica constituting the crucible. Oxygen incorporated in the silicon melt is released as SiO gas from the liquid level of the silicon melt. At the beginning of solidifying, oxygen is incorporated into the silicon melt from the bottom and a side wall of the crucible to increase the oxygen content in the silicon melt, resulting in a higher oxygen content at the bottom which is the starting point of solidification. As the solid-liquid interface moves up with progression of solidification from the bottom, oxygen becomes incorporated only from the side wall, and therefore, there is a gradual decrease in the oxygen content incorporated into the silicon melt. That is why the oxygen content is higher at the bottom which is the starting point of solidification.
So, in the past, for example, as disclosed in Patent Document 2, there has been provided various techniques for controlling incorporation of oxygen by using a silica crucible with a Si3N4 coating layer formed on the inside (the side wall and the bottom) thereof.
Further, when unidirectionally solidifying the polycrystalline silicon ingot, as disclosed in Non Patent Document 1, it has been designed to set a constant solidification rate of, for example, 0.2 mm/min (12 mm/h) to improve its production efficiency.